you just created the client, you haven't logged in with your token yet, this is why "client.channels.cache" is empty and contains no channels yet. To fix this you have to put everything on the "ready" event

Also there's some other problems on your code:

1. You have to async/await the "sleep" function or it's useless
2. On the if conditions, you have to check with === or ==, not = see here why
3. 1000 is way too short (1000 millisecondes is 1 second) posting 1 message/s is api request spam. Try using 4 or 5 seconds instead.

const { SlashCommandBuilder } = require('@discordjs/builders');
const { Client, Collection, Intents, MessageEmbed } = require('discord.js');

const client = new Client({ intents: [Intents.FLAGS.GUILDS, Intents.FLAGS.GUILD_MEMBERS] })

client.on('ready', async () =>{
  const channel = client.channels.cache.get('997960087249371136');

  do {
    await sleep(1000)
    let number = Math.floor(Math.random() * 5);
  
    if(number == 1) return channel.send("Carrier under attack!")
    else if(number == 2) return channel.send("Carrier leak detected in lower quaters!")
    else if(number == 3) return channel.send("All is quiet, for now.")
    else if(number == 4) return channel.send("Unidentified aircraft entering our airspace!")
    else return channel.send("The impossible is possible!")
  } while(true);
})

function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

client.login("YOUR TOKEN")